1. Field of the Invention
The present invention relates to the technology field of a ball screw, and more particularly to a circulation passage structure for a ball screw, wherein the radius of curvature of the return passage of the circulation passage structure is comparatively small and the cross section of the return passage is formed in the shape of an oval, so as to facilitate the passage of the balls.
2. Description of the Prior Art
In order that the balls inside a ball screw can roll smoothly and quietly, a common method is to arrange spacer between neighboring balls, so as to protect the balls from impact and abrasion damage.
The design of the size of the spacer is quite important if want to set the spacer between neighboring balls. If the spacer is too small, the spacer between two neighboring balls is likely to become loose, even worse, the spacer may fall down and jam the balls. If the spacer is too large, the outer periphery of the spacer is likely to contact the circulation passage, causing interference and affecting the operation of the balls. In this case, the size of the spacer is usually designed to be slightly smaller than the diameter of the balls. For example, U.S. Pat. No. 6,742,408 solved the aforementioned problem by restricting the size of the spacer within the range of 0.5-0.9 times the diameter of the balls.
Referring to FIG. 1, which is an illustrative view of showing an outer circulation type ball screw structure, wherein the nut 81 is drilled with a circulation passage 83 that is to be connected to the helical ball channel 82 of the screw shaft by a return pipe 84, thus forming a ball circulating path for the balls 7.
Since the outer circulation ball screw is less restricted by shape and space, the radius of curvature of the return pipe 84 can be designed to be comparatively large. In this way, setting the size of the spacer within the range of 0.5-0.9 times the diameter of the balls can effectively solve the problems of the spacer getting loose, falling down and interfering the balls, and can enable the balls 7 and the spacers 85 to move smoothly in the return pipe 84.
However, the protruding return pipe 84 is prone to be impacted and deformed by external force, causing interference with the balls and the spacers, or even worse, the balls will be jammed. Furthermore, the current trend of the design of the ball screw structure is toward miniaturization and lightweight, the circulation space that the nut can provide for the balls is relatively reduced. The screw shaft of the outer circulation ball screw is unsuitable for miniaturization, and the screw shaft without external structure comparatively meets the requirements for miniaturization and lightweight.
Referring to FIG. 2, which is an illustrative view of a conventional inner circulation ball screw structure, wherein the nut 91 is drilled with a circulation passage 92, and a return piece 93 is arranged at either end of the circulation passage 92. Each of the return pieces 93 has a return path 931 for passage of the balls 7 and the spacers 94, and such structure is disclosed in U.S. Pat. No. 6,176,149.
However, in the miniaturization design, due to the reduction in the outer diameter of the nut, the radius of curvature of the return path 931 of the return piece 93 is also reduced. After the radius of curvature of the return path 931 is reduced, the balls 7 still can roll through it. However, the spacer 94 between two neighboring balls 7 passes through the return path 931 not in a rolling manner but moves in manner of being clamped between two neighboring balls. Therefore, the spacer 94 inside the return path 931 will be pushed to move inward and will contact the inner periphery of the return path 931, as a result, the balls will be squeezed or even jammed, as shown in FIG. 3.
No technology, in the ball screw field, has been proposed to solve the problem of “the spacer contacting the inner periphery of the return path”. However, some people in the technology field of the linear guideway came up with related solutions. For example, U.S. Pat. No. 6,513,977 discloses such a design of forming a chamfer at the conjunction between the return path and the linear path of the circulation path of a linear guideway, such that the interference will not occur when the spacers move from the linear path into the return path.
However, the design of the chamfer is only able to prevent the spacers from impacting the conjunction between the linear path and the return path, the spacers are still likely to contact other portions of the circulation path and cause interference. Therefore, the same applicant proposed an improved design, as disclosed in U.S. Pat. No. 6,663,285, which aims at improving the inner periphery of the return path of the linear guideway by reducing the radius of curvature of the inner periphery of the return path, the entire inner periphery of the return path retracts inward, so as to prevent the spacers from contacting the inner periphery of the return path.
It should be stressed again that the disclosures of U.S. Pat. Nos. 6,513,977 and 6,663,285 are all about linear guideway structure, and the structure used in the linear guideway will probably be unapplicable to the ball screw.
Because the return path of the linear guideway provides enough space, the return path of a linear guideway can be designed in the shape of a semicircle that has a relatively large radius of curvature, so that the semicircular return is able to joint two parallel linear paths together. Therefore, the design of the inner periphery of the return path of a linear guideway can take the form of concentric radius reduction.
However, in a ball screw, the return path is usually designed in an cassette, as shown in FIG. 4, and the cassette 6 must be designed to have a linear guide path 62, 63 arranged at both ends of the return path 61 for connecting the helical groove 64 of the nut and the return passage 65 parallel to the axial direction of the nut. Therefore, the angle between both ends of the return path 61 will be smaller than 90 degrees. In this case, if applying the method of concentric radius reduction disclosed in the U.S. Pat. No. 6,663,285 to the cassette of a ball screw, a level difference G will exist between the guide path 62, 63 of the return path 61 and the helical groove 64 and the return passage 65, as shown in FIG. 5, causing interference with the circulation of the balls, or even worse, the balls will be jammed. Therefore, the method of concentric radius reduction disclosed in the U.S. Pat. No. 6,663,285 cannot be used in the ball screw to solve the problem of the spacer contacting the inner periphery of the return path.
The present invention has arisen to mitigate and/or obviate the afore-described disadvantages.